Many processes have been proposed for the treatment of municipal sewage, and a number of them are in operation. The purpose of such treatment is essentially to separate the solids, whether dissolved or suspended, from the water in which they are carried, so that the residual water, adequately clarified, disinfected and generally rendered harmless to the environment, can be returned to the streams and rivers.
It is known to use lime for the treatment of sewage, usually as one step in a multistep operation. The addition of lime is remarkably effective for the precipitation and the removal of phosphates, which are usually present in abundance in the municipal waste water; it aids in the settling or organic solids, primarily suspended solids; it reduces the COD and BOD of the residual water; and also greatly reduces the number of bacteria and kills viruses and other micro-organisms usually present in waste waters. Lime treatment is sometimes used in primary treatment of sewage, and is then followed e.g. by biological treatment of the residual waters or by treatment with activated carbon. In other cases lime is used in tertiary stage treatment following conventional primary and secondary stages (Lake Tahoe).
It has also been shown that a two-stage lime treatment, where the first stage consists of treatment with lime to a pH greater than 11, followed by clarification; and the second stage consists of adjustment to a pH less than 9.5, e.g. by carbonization, followed by a secondary clarification stage. This two-stage treatment will reduce the concentration of phosphates and magnesium ions in the effluent. However, such lime-treated effluents may still contain a certain amount of bacteria.
It is known to chlorinate waters of various types, in particular the effluent from primary and secondary treatment, to reduce the bacteria content of the water and render it biologically safe. However, when such treated effluents from industrial or municipal waste are treated with chlorine, toxic or otherwise undesirable compounds are formed, e.g. chloramines from the reaction of chlorine with ammonia or chlorinated hydrocarbons as a result of reaction with organic compounds.